With the recent tendency toward ultra-large-scale integrated circuits (ULSI), studies are under way to develop (Ba, Sr)TiO.sub.3 films having high dielectric constants as capacitors and Ru and/or RuO.sub.2 films are frequently employed as electrodes thereof. It has been a practice to produce these Ru films by the Ru metal sputtering method, while the RuO.sub.2 films are produced by the reactive sputtering method of Ru metal in many cases. In further microscaled cases, however, it is expected that the desired step coverage or mass-productivity can be achieved by the CVD method. As the volatile ruthenium compounds to be used in the CVD method, investigations are in progress on tris(dipivaloylmethanato)ruthenium Ru(dpm).sub.3 and bis(cyclopentadienyl)ruthenium Ru(C.sub.5 H.sub.5).sub.2. Nakabayashi et al. reported that an Ru film was formed on an Si substrate at 600.degree. C. by using Ru(dpm).sub.3 by the CVD method and then oxygen was introduced thereinto to give an RuO.sub.2 film on the Ru [Proceedings of the 55th Symposium of Applied Physics in Autumn of 1994, p. 347, 19p-M-9 (1994)]. However, Ru(dpm).sub.3 has a melting point of 168.degree. C. and thus occurs as solid crystals at room temperature. Because of having a vapor pressure of 0.1 Torr at about 136.degree. C., it is supplied via sublimation.
D. E. Trent, B. Paris and H. H. Krause reported that a ruthenium metallic specular film with a purity of 99.99% was formed on a vycor glass substrate at 595.degree. C. by the CVD method by supplying the sublimated vapor from Ru(C.sub.5 H.sub.5).sub.2 maintained at 94.degree. C. together with hydrogen gas [Inorg. Chem., 3, 1057 (1964)].
W.-C. Shin and S.-G. Yoon reported that Ru(C.sub.5 H.sub.5).sub.2 was subjected to CVD at about 350.degree. C. in an oxygen gas to thereby form an RuO.sub.2 film of 200 nm on an SiO.sub.2 /Si, MgO substrate [9th International Symposium on Integrated Ferroelectrics (Santa Fe, N. Mex. March, 1997), p. 104].
U.S. Pat. No. 5,130,172 has disclosed a process for coating a substrate with a metal comprising: maintaining the substrate at a temperature up to 190.degree. C.; exposing this substrate to a vaporized organometallic compound represented by the formula L.sub.n MR.sub.m obtained by heating to a temperature up to 100.degree. C.; then exposing the substrate to a hydrogen gas at a temperature up to 100.degree. C.; and reacting the organometallic compound with hydrogen to thereby form a metal film. In the above formula L.sub.n MR.sub.m, L is hydrogen, ethylene, allyl, methylallyl, butadienyl, pentadienyl, cyclopentadienyl, methylcyclopentadienyl, cyclohexadienyl, hexadienyl, cycloheptatrienyl or a derivative of these compounds having at least one alkyl side chain having less than five carbon atoms; M is a metal that can readily cycle between two oxidation states and can catalyze hydrogenation of hydrocarbon ligands; R is methyl, ethyl, propyl or butyl; n is an integer from 0 to the valence of the metal; m is an integer from 0 to the valence of the metal; and m plus n must equal the valence of the metal. The cyclopentadienylruthenium compounds given in the claims specifying the same are cyclopentadienyl(methylcyclopentadienyl)ruthenium (C.sub.2 H.sub.5)Ru(C.sub.5 H.sub.4 CH.sub.3), ruthenocenylacetylene (C.sub.5 H.sub.5)Ru(C.sub.5 H.sub.4 CCH), ethenylruthenocene (C.sub.2 H.sub.5)Ru(C.sub.5 H.sub.4 CHCH.sub.2), bis(methylcyclopentadienyl)ruthenium Ru(C.sub.5 H.sub.4 CH.sub.3).sub.2 and ethylruthenocene (C.sub.5 H.sub.5)Ru(C.sub.5 H.sub.4 CH.sub.2 CH.sub.3).
Table 1 shows the melting point of each of the compounds cited above shown in Dictionary of Organometallic Compounds, vol. 3 (2nd Ed., 1966, Chapman & Hall). Namely, all of these compounds except ethylruthenocene are solids at room temperature of 25.degree. C.
TABLE 1 M.p. (.degree. C.) bis(cyclopentadienyl)ruthenium Ru (C.sub.5 H.sub.5).sub.2 199-200 bis(methylcyclopentadienyl)ruthenium Ru (C.sub.5 H.sub.4 CH.sub.3).sub.2 61-63 cyclopentadienyl(methylcyclopentadienyl)- 41-42 ruthenium (C.sub.5 H.sub.5) Ru (C.sub.5 H.sub.4 CH.sub.3) ethenocenylacetylene (C.sub.5 H.sub.5) Ru (C.sub.5 H.sub.4 CCH) 73-74 ethenylruthenocene (C.sub.5 H.sub.5) Ru (C.sub.5 H.sub.4 CHCH.sub.2) 53.5-54.5 ethylruthenocene (C.sub.5 H.sub.5) Ru (C.sub.5 H.sub.4 CH.sub.2 CH.sub.3) 12-12.5
The supply of the starting compound via sublimation in the CVD method is inferior in quantitative supply, controllability and mass-productivity to the liquid supply system or evaporation supply system with the use of a carrier gas bubbling into the liquid. Accordingly, it is required to employ a starting compound which is a liquid at the step of supply at room temperature and has a sufficient vapor pressure. Moreover, it is necessary that the starting compound can be easily produced on a mass scale. Among the known cyclopentadienylruthenium compounds capable of forming an Ru film by the CVD method, none but ethylruthenocene is a liquid at room temperature of 55.degree. C. and has a vapor pressure.
However, ethylruthenocene is poor in mass-productiveness. According to V. Mark and M. D. Raush ethylruthenocene is synthesized by reducing cyclopentadienyl(acetylcyclopentadienyl)ruthenium Ru(C.sub.5 H.sub.5)(C.sub.5 H.sub.4 COCH.sub.3) with LiAlH.sub.4 +AlCl.sub.3 in diethyl ether [Inorg. Chem., vol. 3, 1067 (1964)]. Namely, it is necessary to use a specific cyclopentadienylruthenium compound as an intermediate, which brings about problems in mass-productiveness and production cost. That is to say, there has been known so far no cyclopentadienylruthenium compound which can be easily produced on a mass scale, is a liquid at room temperature and has a sufficient vapor pressure.
An object of the present invention is to specify cyclopentadienylruthenium compounds which are liquids at room temperature of 25.degree. C., have sufficient vapor pressures, can be easily produced on a mass scale and are usable in forming Ru and RuO.sub.2 films by the CVD method and provide a process for forming Ru and RuO.sub.2 films by the CVD method with the use of these compounds. Another object of the present invention is to provide a process for producing these specific compounds being excellent in mass-productivity.